Inches but I would try to move as much dirt as I can lift. Using the dimensions of the original or a similar pot & some mulch to draw a circle, I would make the circle 1' longer or more. The roots have not grown thaat much but 1' is easy to picture and then dig out.

Just last spring - aka, 2017. Going 1 foot out past the 15-18 inches diameter of a 3G pot, you might find it lifts fairly easily regardless assuming you just put the plug into the soil. IME, the plugs are a fairly loose mix. Try to get the whole root ball including the new growth